The conventional cupola is essentially a shaft furnace. At the bottom of the shaft is a well portion for collecting the molten metal and for initially receiving a bed charge coke. Closely spaced above the well are tuyeres for feeding large volumes of air under pressure. In the upper portions of the shaft there is provided a charge port. A cupola is employed in metal melting as opposed to metal refining processes.
Normal cupola operation is essentially simple. The vertical shaft furnace is packed with coke, which is caused to burn by air forced in the bottom through the tuyeres, producing heat. Metal, placed on top of the glowing coke bed, melts and drips through the coke, collecting in the well or hearth, where it is removed periodically through a tap hole.
When the incoming air, referred to in the art as the air blast, comes in contact with the burning coke, the latter is burned to carbon dioxide. This immediately reacts with further coke to form carbon monoxide, but in so doing absorbs about 45% of the heat emitted by the original carbon dioxide combustion reaction. As the carbon monoxide ascends through the column of coke and becomes cooler, some of it decomposes to carbon dioxide and carbon, an exothermic reaction.
The gases discharged from the shaft are thus a mixture of carbon monoxide, carbon dioxide and nitrogen. These hot discharged gases carry out about 10 percent of the heat produced by combustion of the coke. About 45 percent of the heat produced is removed by the molten metal, and the remaining 45 percent of the heat produced is used up by the afore-mentioned incomplete combustion reaction.
Those skilled in the art have devised several methods to alleviate the inefficiencies caused by this incomplete combustion. One such method has been to enrich the incoming air with oxygen. This method has given good results, but it is characterized by oxygen loss through leaks and some loss in the control of the chemistry of the molten metal.
Another method which has found wide use in the industry is the injection of extra oxygen directly into the burning coke. When oxygen is introduced in this manner, combustion is much more rapid near the hearth and the length of the zone of combustion tends to be less than with air alone. This causes the top of the coke bed to be somewhat cooler and this in turn causes a correspondingly greater decomposition of carbon monoxide to carbon dioxide and carbon, accompanied by a greater release of heat. This produces a hotter metal, a reduction in the amount of coke required per ton of metal and a higher carbon content in the metal. One such method is disclosed in U.S. Pat. No. 3,089,766 in which oxygen is injected directly into one or more tuyeres at velocities greater than that of the air blast. Another method is disclosed in British Pat. No. 914,904 in which oxygen is injected into the furnace through tuyeres located below the tuyeres through which air is introduced. Still another method is disclosed in British Pat. No. 1,006,274 in which oxygen is injected into the furnace through tuyeres located at the same level as the tuyeres through which air is introduced but in such a manner that the jets of air and oxygen impinge on different areas of the coke charge without substantial intermixing.
Because of the significant economic importance of the metal melting operation in cupolas a method which would improve the efficiency of the process over that heretofore obtained would be highly desirable.